Molecular machines are nano-scale assemblers that construct themselves and their surroundings into ever more complex structures. Sometimes dubbed "nanotech" in the media, these devices are promising — but also widely misunderstood. Here's what separates the science fact from science fiction.The concepts that underpin this form of nanotechnology have certainly had long enough to percolate through modern science. Richard Feynman first speculated about the idea of "synthesis via direct manipulation of atoms" during a talk called There's Plenty of Room at the Bottom. Looking back, that sparked much of the subsequent thinking about treating atoms and molecules more and more like simple building blocks.

Perhaps most famously, K. Eric Drexler considered the idea of taking the bottom-up manufacturing approach to its atomic extreme in his 1986 book Engines of Creation: The Coming Era of Nanotechnology. There, he posited the idea of a nan-oscale "assembler" that could scuttle around, building copies of itself or other molecular sized objects with atomic control; one which might in turn be able to create larger and more complex structures. A kind of microscopic production line, building products from the most basic ingredients of all. Coming when it did, in the mid-eighties, it felt very much like science fiction.

Drowning in grey goo

So much so, in fact, that even Drexler acknowledged that it was prudent to tread carefully in a nano-scale building site. "Imagine such a replicator floating in a bottle of chemicals, making copies of itself," he explains in Engines of Creation. "The first replicator assembles a copy in one thousand seconds, the two replicators then build two more in the next thousand seconds, the four build another four, and the eight build another eight. At the end of ten hours, there are not thirty-six new replicators, but over 68 billion. In less than a day, they would weigh a ton; in less than two days, they would outweigh the Earth; in another four hours, they would exceed the mass of the Sun and all the planets combined — if the bottle of chemicals hadn't run dry long before."That ruthless efficiency could, Drexler argued, make some nano-robots "superior" to naturally occurring organic beings, at least in an evolutionary sense—though, crucially, not necessarily as valuable. Indeed, he suggested that omnivorous bacteria could out-compete real bacteria, reducing the biosphere to dust—or 'grey goo'—in a matter of days. That hypothetical end-of-the-world scenario, where nanobots turn our world and us into an amorphous sludge, was as tempting to skeptics as the promise of nanotechnology was to scientists. Still, almost thirty years on we're still here and, while some of us may be a little more ashen of face, we're yet to be submerged in the biological by-product of engineered molecular machines.

No assembly required

But unlike Lego, when combined in solution the DNA can form structures without intervention. The interactions between strands are controlled by their base sequences: there's a preference for certain sites to bind and others not, resulting in a glorious self-assembly process. If Drexler's suggestions of assemblers felt like science fiction in the mid-eighties, then the fact that molecules can be designed to build themselves into new and complex structures surely renders it fact.

Indeed, Seeman's lab has a rich history of creating self-assembling complex objects such as crystals out of a simple puddle of DNA molecules. His labs have created 2D and 3D crystals, as well as a wide variety of geometric shapes using the techniques. There are plenty of other researchers working in the area, too. Professor Andrew Turberfield at the University of Oxford, for instance, uses DNA molecules to create individual tetraheada, such as the one pictured above. Mixing four different types of DNA, each designed to join together in pre-defined ways, his researchers can create tetrahedra with 7 nanometer edges. They can be used to lock proteins inside the structure, to be deployed to an area where medical treatment is required—a kind of assembling and self-deploying container system at the molecular level.

Make your move, molecule

But the slew of nano-engineered molecules don't just assemble themselves—they move, too. A number of research groups have created molecules that can walk, much like humans or animals. Synthesised from DNA, they're supposed to walk directionally along a track, though until recently it's been difficult to accurately gauge whether the walkers had 'jumped' or 'floated' to a new location—because the steps they take, at around a nanometer in length, are hard to detect using traditional techniques. Fortunately, researchers from Oxford University's Department of Chemistry have now laced the walker's DNA with arsenic, and are able to track it as it leeches through a porous track—proving once and for all that the walkers do indeed do as they're supposed to.Elsewhere, mechanical engineering is a large influence on construction—hence the popular moniker molecular machines. Motors, for instance—one of the life-size objects we largely take for granted—have assumed molecular form. The first molecular motor was made in 2021; the fastest yet produced just last year. The smallest simply spins about a sulphur atom while sat atop a clean copper surface—reaching speed of up to 7,200 revolutions per minute. The fastest, made of a rather more bulky three molecular components, can reach speeds of 18,000 revolutions per minute—roughly the same speed as a jet engine.Perhaps the most complete example of a molecular machine so far, though, is the nanocar, developed by a team of Netherlands-based researchers. Made up of a long central body with pivoted paddles at each of its four corners, a pulse of electrons can be used to swing the paddles around in circles, a whole quarter turn at time. That quarter turn puts the molecule in to an unnatural arrangement, so the bonds continue to move another quarter turn to reach a state of equilibrium. To keep the car moving requires a pulse of electrons every half turn. It's perhaps not setting any distance records—it takes 10 pulses of electrons to move the vehicle 6 nanometers—but it is a molecular car. Give it a break.

Alternative energy

All this raises an easily overlooked problem though: how do we power these molecular machines? "The major challenge so far in the field is still the propulsion of synthetic nanomotors," explains Dr Wei Gao, from the Department of Electrical Engineering & Computer Sciences at the University of California, Berkeley. "New nanomotors which can travel inside living animals, especially the bloodstream, efficiently are still strongly desired."